Antarctic meteorite points to smashed dwarf planet

Lost world of early solar system

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Two pieces of meteorite found in 2006 in the Graves Nunataks area of Antarctica are likely fragments of a dwarf planet destroyed in the solar system's youth, New Scientist reports.

Detailed studies of the chunks, dubbed GRA 06128 and GRA 06129, significantly revealed that they contain 75-90 per cent feldspar, indicating their parent body underwent "differentiation" in which dense material settles towards the centre of molten, magma ocean-lapped bodies in their first few tens of millions of years.

Samples of Moon rock show even higher levels of "relatively lightweight" feldspar, "thought to be the result of crystals of feldspar solidifying from the early magma ocean" and floating to the top "allowing it to form a highly concentrated layer of the mineral".

Accordingly, Allan Treiman of the Lunar and Planetary Institute in Houston, Texas, who studied one of the rocks, said that "the amount of feldspar in the two meteorite fragments suggests they are remnants of a very large body that differentiated in a similar way".

This conclusion is backed by other studies of the meteorite remains, including one headed by Richard Ash of the University of Maryland in College Park, a second led by Chip Shearer of the University of New Mexico, and a third directed by Ryan Zeigler of Washington University in St Louis, Missouri. All concur that "the parent body must have been massive enough to have separated into layers".

The University of Maryland team also ruled out the possibility that GRA 06128 and GRA 06129 were bits of the Earth, Mars, Moon or Venus. A peruse of the radioactive decay of elements they contain showed that they "must have formed around 4.5 billion years ago, when Earth and the other planets were coalescing".

Treiman, who led yet another study of one of the fragments, told New Scientist: "This is a piece of a dwarf-planet size body that apparently no longer exists. We have here a sample of a strange new world, a sample we've never seen before."

Treiman reckons the feldspar concentrations "suggest that body was probably smaller than the 3,500-kilometre-wide Moon but larger than Vesta, the third largest asteroid in the solar system at 578 kilometres across".

The evidence to back this calculation comes from meteorites believed to come from Vesta which "contain solidified lava, but not large concentrations of feldspar, suggesting its gravity was not strong enough to form a distinct layer of the mineral".

Quite how the parent body of GRA 06128 and GRA 06129 disappeared is unknown. A collision may have resulted in fragments which still haunt the solar system as asteroids and "might be identified by their light spectra".

The teams'* research findings were presented yesterday at the Lunar and Planetary Science Conference in Houston, Texas. ®

Bootnote

*A fifth and final team from National Institute for Polar Research in Tokyo provocatively suggested that some of the meteorite's characteristics, including "the high abundance of sodium in some of its minerals", hint at a parent body rich in water.